The present invention relates to an image sensor for use in facsimile devices, image readers, digital scanners and so on. More particularly, the invention relates to an image sensor comprising, as formed on one and the same substrate, a plurality of photoelectric energy converting elements, switching elements connected to said photoelectric converting elements, and a matrix wiring connected to said photoelectric converting element. In this specification the term "substrate" is sometimes replaced by "substrate board" or merely "board".
Prior Art No. 1 and its Problems
The image sensor heretofore used for a facsimile device, image reader, digital scanner or the like comprises a reducing optical system and a CCD (charge coupled device) type sensor. However, the recent development of thin-film semiconductor materials represented by hydrogenated amorphous silicon (hereinafter referred to briefly as a-Si:H) enabled the industry to form a plurality of photoelectric converting elements, switching elements, etc. on an elongated substrate board and thereby provide an extra-length image sensor capable of reading a document with isometric optics. As a result, amazing progresses have been made in the development of extra-length contact-type sensor devices utilizing such elongated image sensors.
With attention paid to the fact that, among various thin-film semiconductor materials, a-Si:H, in particular, can be used not only as a semiconductor material for photoelectric energy conversion but also as a semiconductor material for diodes, there was proposed an image sensor using diodes as switching elements and comprising a photoelectric converting element semiconductor layer and a diode semiconductor layer both formed from a-Si:H on the same substrate board, with a matrix wiring also formed on the same board (e.g. Japanese Kokai Patent Publication No. 56-1318 and No. 58-56363). Moreover, for accelerating the reading speed and realizing high-tone reading, there has been proposed a picture image sensor using PIN type photodiodes of a-Si:H as photoelectric energy converging elements and comprising diodes and matrix conductors on the same substrate board.
These prior art image sensors are comprised, as shown in FIG. 19, of a glass substrate board 1, a first electrode 2 of chromium Cr, blocking diodes 3 and photodiodes 4 which are connected in series and opposite polarity through said first electrode 2, and matrix conductors 5. Here, the photodiodes 4 and blocking diodes 3 are formed of the same amorphous silicon semiconductor layer and the photodiodes 4 and matrix conductors 5 are connected to each other by second electrodes 8 of chromium Cr through contact holes 7 formed in a transparent interlayer insulating film 6 of SiO.sub.2, with the plurality of blocking diodes 3 being interconnected.
While this image sensor is so constructed that photodiodes 4 and blocking diodes 3 are connected in series and opposite polarity through the first electrodes 2 formed on the glass substrate board 1, the longitudinally extending matrix conductors 5 are, of necessity, formed simultaneously with said first electrodes 2 as seen from FIG. 19. However, the raw material for this first electrode 2 must be selected with attention to adhesion to the glass substrate board 1 and prevention of diffusion of metallic impurities into the amorphous silicon semiconductor layer. The preferred electrode materials from the standpoint of preventing diffusion of metallic impurities are metal materials such as chromium Cr, titanium Ti, nickel Ni and so on. On the other hand, from the standpoint of adhesion to the glass substrate board 1, the first electrode 2 and matrix wiring 5 are desirably as thin as possible ad this is also instrumental for reducing the steps formed in the transparent interlayer insulating film 6 and so on. However, in order to reduce the wiring resistance, the thickness of said first electrode 2 and matrix wiring is preferably thick enough. For this reason, if chromium Cr is used as a raw material for said first electrode 2 and matrix wiring 5, the resistance of the elongated matrix wiring 5 will be increased to increase the reading time constant in the reading of sensor electric signals, thus detracting from the high-speed reading performance of the image sensor.
The image sensor is so constructed that signals are serially read and processed in unit blocks arranged in a time-series. In the reading of such signals, the signal read by a photodiode in the (m)th block is affected by the corresponding photodiode in the immediately preceding (m-1)th block. This influence is significant in a device for reading multiple-tone colors. However, the photodiode in the (1)st block is not subject to the influence of other blocks, hence there is the problem that the signal output value from this block is different from the output values from the other blocks. Furthermore, the image sensor comprising photodiodes and blocking diodes formed by deposition of amorphous silicon semiconductor layers, for instance, tends to vary from one unit to another in the thickness of the built-up semiconductor layer so that these units are varying, though slightly, in characteristics.
An exemplary reading action in an image sensor is now described. First, an electric charge is accumulated in the photodiode, this accumulated charge is then released for a given time period according to the quantity of received light, and finally a driving pulse is applied to the photodiode for recharging to compensate for the amount of discharge. The integral value of current is read for use as a read signal. When the size of the blocking diode is sufficiently large with respect to the photodiode, the electric charge released from the photodiode is rapidly replenished so that the reading speed is high. However, the size of the blocking diode is determinant of the capacity of the blocking diode and there was the problem that when this capacity is large, the noise is produced by a capacitance kick in dark output so as to lower the signal-to-noise ratio.
Second prior art and its problem
The linear image sensor comprises a glass substrate board, a linear array of a plurality of semiconductor elements each consisting of a lower electrode, a semiconductor layer and an upper electrode, an inter-layer dielectric film covering and insulating said plurality of semiconductor elements, and a wiring disposed through said interlayer dielectric film. The size of semiconductor elements and conductors constituting this linear image sensor is of the order of .mu.m and the photolithographic technique is generally used for microfine processing. Since, in the manufacturing of a linear image sensor, the processes of film formation, photoresist coating, photomask positioning, exposure to light, and etching must be performed repeatedly, it is most advantageous to fabricate a large number of linear sensors in one production setup for reducing the cost of production.
To this end, it is common practice to form a plurality of linear image sensors on a glass substrate board having a large surface area and, then, cut the glass board with a dicer which is commonly used in the production of silicon wafers and the like to provide discrete linear image sensors. Here, a standard-sized glass substrate board having a thickness of 1.1 mm, which is easily available at low cost and easy to use, is generally employed. Then, as shown in FIG. 20, a resin film 103 is bonded to the reverse side of the glass substrate board 102, which carries semiconductor elements 101 and others on its face side, with an adhesive agent 104, and using the rotary cutter 105 of the dicer, the glass board 102 is serially cut to size, leaving the resin film 103. By the provision of this resin film, the boards 102 of individual linear image sensors 106 are prevented from entrapment by the rotary cutter 105 of the dicer or falling down to break.
For this cutting of the glass substrate board 102 with the dicer, there must be a cutting margin A, which is generally at least 0.2 mm, as shown in FIG. 21. When the width of the linear image sensor 106 in the subordinate scanning direction is 2 mm, for instance, the cutting margin A amounts to at least 10% of this dimension so that the number of unit linear image sensors 106 that can be obtained from one glass substrate board having a given area is as much limited. Moreover, since the glass is subject to chipping by the rotary cutter 105 at both sides B of the cutting line, there cannot be obtained sharp corner faces so that when the product linear image sensor 106 is built into a module, the positioning becomes difficult. Moreover, a margin to allow for this chipping B is required in the width of the glass substrate board 102 in the subordinate scanning direction, with the result that the number of linear image sensors 106 that can be obtained form a glass substrate of given dimensions is further limited.
Furthermore, the cutting of the glass substrate board 102 with the dicer is a time-consuming operation and is a rate-determining step of production. In addition, while the cutting length of the glass substrate board 102 depends on the reading size of the linear image sensor 106, it is generally about 200 to 340 mm and it requires a special long dicer to cut out such extra-length sensors. However, partly because such a long dicer has limited uses, it cannot be easily obtained and is expensive. Moreover, the dicer requires periodic change of its cutter 105 and, hence, entails a high running cost.
Moreover, while the resin film 103 is bonded to the reverse side of the blank glass substrate board 102 so that the cut board 102 will not be entrapped by the revolving cutter as mentioned before, this resin film 103 must be removed after completion of the cutting operation. This procedure involves dipping of the sensors in a solvent, for instance, but such treatment tends to damage the preformed semiconductor elements 101 and other components on the glass board 102.
Third prior art and its problems
This linear image sensor comprises a glass substrate board, a linear array of semiconductor elements each consisting of a lower electrode, a semiconductor layer and an upper electrode as formed on said substrate board, an interlayer dielectric film covering and insulating said plurality of semiconductor elements, and a wiring connected through said interlayer dielectric film. The film thicknesses of these semiconductor elements and conductors constituting this linear image sensor range from 800 to 1600 Angstrom units and their sizes are of the order of .mu.m. For this reason, when, as shown in FIG. 22, a chromium film 202, which serves as a lower electrode, and an ITO film are formed on a glass substrate board 201 having an uneven surface due to curling or marring, a pinhole 203 tends to form around a convex or concave of irregularity. If a pinhole 203 is present, disconnection occurs at the pinhole 203 in patterning or the wiring resistance will be increased. Moreover, when the surface of the glass substrate board 201 is uneven or marred, the surface irregularity remains uncorrected even after super-imposition of a metal electrode film 202 as seen from FIG. 23, with the result that in the spin coating of photoresist 204 at the photofabrication stage, the photoresist 204 fails to adhere to the area 205 behind the recess or there occurs a difference in thickness of the photoresist film 204 between the convex and concave parts, thus leading to uneven exposure and poor patterning.
Furthermore, the linear image sensor for A4 size paper must have a length in excess of 200 mm in the main scanning direction and the electrode pads necessary for driving and reading are arranged along the entire length. At the testing of electrical characteristics, as shown in FIG. 24, the probing styluses 206 are contacted with the electrode pads arranged one-dimensionally. Therefore, when the glass substrate board 201 has a curl, an offset takes place between the probing stylus 206 and the electrode pad and, because of the varying height of the electrode pads, some styluses 206 may not contact the electrode pads, with the result that no proper test data can be generated. Moreover, there is the risk that in the spin coating of the photoresist, such a curled glass substrate board cannot be adsorbed by vacuum suction. In the exposure step, too, no uniform focusing can be accomplished. In addition, when a linear image sensor fabricated with such a curled glass substrate board 201 is bonded to a supporting board or a printed circuit board, the resulting linear stretching of the board 201 tends to damage the preformed semiconductor elements and conductors on the glass board. Moreover, fixing a curved glass substrate board for dicer cutting requires some special contrivance, while the scribe-break method has a drawback in that because the transmission of the bending force is irregular, the glass substrate cannot be broken at right angles with its surface.
Furthermore, when an amorphous silicon type semiconductor is used for the semiconductor layer forming a semiconductor element, chromium is generally used for the metal electrode layer so that metal atoms will not diffuse into this semiconductor layer. However, alkali tends to emigrate from the glass substrate beneath this metal electrode layer to form sodium chromate. Since this sodium chromate is water-soluble, there occurs the problem that pinholes are produced in the metal electrode layer.